1. Field of the Invention
The present invention relates to a two stage utterance verification device and a method thereof in a speech recognition system, and more particularly, to a two stage utterance verification device in a speech recognition system for improving reliability of utterance verification by performing two stages of utterance verifications where a first stage of utterance verification is performed based on a support vector machine (SVM) pattern classification method by using data outputted from a search unit of a speech recognizer and a second stage of utterance verification is performed based on a classification and regression tree (CART) pattern classification method by using data outputted from functional blocks of speech recognizer, and a method thereof.
2. Description of the Related Art
Speech recognition has been utilized for controlling a terminal or an electric home appliance or for accessing target information. Recently, the speech recognition has been further implemented to an intelligent robot, a telematics and a home network. A speech recognition system should recognize 100% of speech uttered from a user. However, some of vocabularies are misrecognized by various factors such as noises, mistakes of a user and uttering of vocabularies which are not a target of recognition (OoV: out of vocabulary (OoV).
If a speech is misrecognized by the speech recognition system and the misrecognized result is transferred to the user as a system response, the user may have difficulty to use the speech recognition system. That is, it degrades usability of the speech recognition system. Therefore, a rejection function has been introduced. The rejection function performs an utterance verification method for verifying the speech recognition result. If the rejection function determines that the speech recognition result is confident, the rejection function transfers the result of speech recognition to the user as the system response. In contrary, if the rejection function determines that the speech recognition result is not confident, the rejection function requires the user to utter corresponding speech again.
As mentioned above, the utterance verification method analyzes a reliability of the speech recognition result by determining a possibility of misrecognizing corresponding speech and a possibility of correctly recognizing corresponding speech. According to the analyzing result, the utterance verification method decides the rejection of the speech recognition result. The utterance verification method can incredibly increase the usability of the speech recognition system even in a vehicle environment or a home environment. Therefore, the utterance verification method has been a necessary function of the speech recognition system.
A conventional speech recognition system uses a support vector machine (SVW) pattern classification method as the utterance verification method. The SVM pattern classification method determines misrecognition of the speech based on feature data such as an anti-model log likelihood ratio (LLR) score, N-best LLR score, combination of LLR score and word duration which are outputted from a search unit of a hidden markov model (HMM) speech recognizer.
Because the SVM pattern classification method of the conventional speech recognition system mainly uses intermediate results outputted in a recognition candidate searching step, the reliability of utterance verification is degraded and the conventional speech recognition system cannot provide satisfactory speech recognition service to the user.